Microstructural characteristics and low-cycle fatigue properties of AZ91 and AZ91-Ca-Y alloys extruded at different temperatures

The commercial AZ91 alloy and nonflammable SEN9(AZ91-0.3Ca-0.2Y,wt%)alloy are extruded at 300°C and 400°C.Their microstructure,tensile and compressive properties,and low-cycle fatigue(LCF)properties are investigated,with particular focus on the influence of the extrusion temperature.In the AZ91 and SEN9 materials extruded at 300°C(300-materials),numerous fine Mg_(17)Al_(12)particles are inhomogeneously distributed owing to localized dynamic precipitation during extrusion,unlike those extruded at 400°C(400-materials).These fine particles suppress the coarsening of recrystallized grains,decreasing the average grain size of 300-materials.Although the four extruded materials have considerably different microstructures,the difference in their tensile yield strengths is insignificant because strong grain-boundary hardening and precipitation hardening effects in 300-materials are offset almost completely by a strong texture hardening effect in 400-materials.However,owing to their finer grains and weaker texture,300-materials have higher compressive yield strengths than400-materials.During the LCF tests,{10-12}twinning is activated at lower stresses in 400-materials than in 300-materials.Because the fatigue damage accumulated per cycle is smaller in 400-materials,they have longer fatigue lives than those of 300-materials.A fatigue life prediction model for the investigated materials is established on the basis of the relationship between the total strain energy density(ΔW_(t))and the number of cycles to fatigue failure(N_(f)),and it is expressed through a simple equation(ΔW_(t)=10·N_(f)-0.59).This model enables fatigue life prediction of both the investigated alloys regardless of the extrusion temperature and strain amplitude.

Fatigue small crack growth threshold determination of a high-Nb TiAl alloy at different temperatures by in-situ observation

The purpose of this paper is to estimate the fatigue crack growth threshold of a high-Nb TiAl alloy at the different temperatures based on scanning electron microscopy （SEM） in-situ observation. The results indicated that the fatigue crack growth threshold △Kth of a nearly lamellar high-Nb TiAl alloy with 8% Nb content at room temperature and 750℃ was determined as 12.89 MPa.m^1/2 and 8.69 MPa.m^1/2, respectively. The effect of the elevated temperature on the fatigue crack growth threshold cannot be ignored. At the same time, the early stage of fatigue crack propagation exhibited multicrack initiation and bridge-link behavior.

Interaction of fatigue and creep of GH33 under multi-axial stress at high temperature

Low-cycle fatigue experiments of tension-compression, torsion andtension-torsion with holding time were performed. The interaction law of creep and fatigue undermultiaxial stress at high temperature was investigated, and the micro-mechanism of equilibriumdiagrams was analyzed. A united equation of fatigue life under multiaxial stress was proposed.

Effect of Cu addition on overaging behaviour,room and high temperature tensile and fatigue properties of A357 alloy

The aims of the present work are to evaluate the overaging behaviour of the investigated Cu-enriched alloy and to assess its mechanical behaviour,in terms of the tensile and fatigue strength,at room temperature and at 200℃,and to correlate the mechanical performance with its microstructure,in particular with the secondary dendrite arm spacing(SDAS).The mechanical tests carried out on the overaged alloy at 200℃ indicate that the addition of about 1.3 wt.%Cu to the A357 alloy enables to maintain ultimate tensile strength and yield strength values close to 210 and 200 MPa,respectively,and fatigue strength at about 100 MPa.Compared to the quaternary(Al−Si−Cu−Mg)alloy C355,the A357−Cu alloy has greater mechanical properties at room temperature and comparable mechanical behaviour in the overaged condition at 200℃.The microstructural analyses highlight that SDAS affects the mechanical behaviour of the peak-aged A357−Cu alloy at room temperature,while its influence is negligible on the tensile and fatigue properties of the overaged alloy at 200℃.

Influence of the Steel-making Dust on High Temperature and Fatigue Performance of Asphalt Mortars

To research the possibility of steel-making dust as a kind of mineral filler in asphalt mixture, two steel-making dusts and one ordinary mineral filler were adopted. The specific density, specific surface area, fineness modulus and mineralogy component of the dusts were tested. Scanning electron microscopy(SEM) was carried out to research the microstructure of the dusts; dynamic shear rheological(DSR) test and time sweep test were used to research the high temperature and fatigue performance of asphalt mortars containing steel-making dust. The experimental results indicate that, compared with ordinary mineral filler, steel-making dusts have more active ingredients, difference surface characteristics and micro-structure. Furthermore, the high temperature and fatigue performance of steel-making dusts corresponding asphalt mortars are superior to those of reference group. Therefore, the steel-making dust would be an alternative to the ordinary mineral filler to improve the performance of asphalt mortars and reduce the harm of the dusts to the environment at the same time.

Effect of alloying on high temperature fatigue performance of ZL114A(Al-7Si) alloy

The effect of Cu,Fe and Ni on high-temperature mechanical performance and fatigue properties of ZL114A alloy was studied through high temperature fatigue test and SEM.The results show that the three elements have a detrimental influence on high temperature cyclic fatigue life.When the contents (mass fraction) of Fe,Cu and Ni in ZL114A alloy are 0.28%,1.53% and 0.16%,respectively,the high temperature tensile strength and cyclic fatigue of ZL114A alloy are improved from 194 MPa and 40.2 MPa to 236 MPa and 48.2 MPa by alloying.The main reason that high temperature tensile strength and cyclic fatigue of ZL114A alloy are improved significantly is that the three elements greatly improve the proportion of Cu/Mg in ZL114A alloy and nickel content.

Effect of Rare Earth Elements on Low Frequency Fatigue of Hot Rolled Steel 60CrMnMo at High Temperature

The effect of rare earth(RE) on low frequency high temperature fatigue strength of hot rolled steel 60CrMnMo was investigated. The results show that by adding a certain amount of RE in steel 60CrMnMo, its serviceable life during low frequency fatigue test at high temperature can be improved.The number of cycles for fracture was increased by 18.5%～28.8%. The number of large inclusion particleswas reduced and the composition of inclusions and their shape were changed as well. The strip distances on fatigue fracture surface were decreased. It is concluded that the optimum amount of RE addtion is about 0.05 wt%～0. 10 wt%.

The Effect of Pre-Thermal and -Load Conditions on IN-718 High Temperature Fatigue Life

Ni-based superalloys are largely used in the aerospace industry as critical components for turbine engines due to their excellent mechanical properties and fatigue resistance at high temperatures. A hypothesis to explain this atypical characteristic among metals is the presence of a cross-slip mechanism. Previous work on the role of thermal activation on cubic slip has shown strain accommodation in two sets of slip planes, which resembled the activation of {100} cubic slip systems along of the octahedral slip planes {111} in Ni-based superalloys under high strain and temperature, exhibiting a more homogeneous strain distribution and less strain localization. Following those previous literature evaluations of initial conditions that can potentially activate cubic-slip planes and provide the level of accommodation and strain homogenization within the grain, this paper presents some experimental procedures and results of Ni-based superalloy (IN-718) tested at 500°C under operational loading condition, without and after being submitted to an overload and overtemperature. The experiments have shown that a pre-condition of 1% strain at 700°C would increase the fatigue life of the IN-718 at 500°C by four times when compared to pristine tested samples. The present results bring up the potential of improving this material fatigue performance, opening the need to further investigate the microstructure as the precondition is applied.

Performances of fissured red sandstone after thermal treatment with constant-amplitude and low-cycle impacts

In the engineering practices,it is increasingly common to encounter fractured rocks perturbed by temperatures and frequent dynamic loads.In this paper,the dynamic behaviors and fracture characteristics of red sandstone considering temperatures(25℃,200℃,400℃,600℃,and 800℃)and fissure angles(0°,30°,60°,and 90°)were evaluated under constant-amplitude and low-cycle(CALC)impacts actuated by a modified split Hopkinson pressure bar(SHPB)system.Subsequently,fracture morphology and second-order statistics within the grey-level co-occurrence matrix(GLCM)were examined using scanning electron microscopy(SEM).Meanwhile,the deep analysis and discussion of the mechanical response were conducted through the synchronous thermal analyzer(STA)test,numerical simulations,one-dimensional stress wave theory,and material structure.The multiple regression models between response variables and interactive effects of independent variables were established using the response surface method(RSM).The results demonstrate the fatigue strength and life diminish as temperatures rise and increase with increasing fissure angles,while the strain rate exhibits an inverse behavior.Furthermore,the peak stress intensification and strain rate softening observed during CALC impact exhibit greater prominence at increased fissure angles.The failure is dominated by tensile damage with concise evolution paths and intergranular cracks as well as the compressor-crushed zone which may affect the failure mode after 400℃.The second-order statistics of GLCM in SEM images exhibit a considerable dependence on the temperatures.Also,thermal damage dominated by thermal properties controls the material structure and wave impedance and eventually affects the incident wave intensity.The tensile wave reflected from the fissure surface is the inherent mechanism responsible for the angle effect exhibited by the fatigue strength and life.Ultimately,the peak stress intensification and strain rate softening during impact are determined by both the material structure and compaction governed by thermal damage and tensile wave.

Dual-System Activation of Ni-Base Superalloy under High Strain and High Temperature

Due to their superior combination of heat resistance, high temperature corrosion resistance, toughness and strength, nickel-based superalloys have become of extensive use in the aerospace industry. This research aims to explain why the fatigue life of Inconel-718 in preconditioned samples had larger fatigue lives than pristine samples. The hypothesis is that preconditioning at 700°C and 1.0% strain could lead to thermal activation of the {100} cubic slip plane alongside the {111} octahedral slip plane, potentially improving fatigue life. Using SEM and EBSD imaging, the microstructure of Inconel-718 samples were characterized before and after preconditioning. The directions of the slip bands that formed following the preconditioning were determined. The result was that the existence of both the cubic and octahedral slip systems was confirmed, leading to the thermal activation hypothesized. The existence of both slip planes was considered to be the reason behind the improved fatigue life due to better strain accommodation within the microstructure. It is suggested that focuses for future research includes conducting in-situ observation of slip activation and the application of preconditioning as a manufacturing method.

Microstructure evolution during heat treatment of Mg-Gd-Y-Zn-Zr alloy and its low-cycle fatigue behavior at 573K

In as-cast Mg?2.1Gd?1.1Y?0.82Zn?0.11Zr(mole fraction,%)alloy,lamellar microstructures that extend from grain boundaries to the interior ofα-Mg grains are identified as clusters ofγ′using a scanning transmission electron microscope equipped with a high-angle annular dark-field detector.Under a total strain-controlled low-cyclic loading at573K,the mechanical response and failure mechanism of Mg?2.1Gd?1.1Y?0.82Zn?0.11Zr alloy(T6peak-aging heat treatment)were investigated.Results show that the alloy exhibits cyclic softening response at diverse total strain amplitudes and573K.The experimental observations using scanning electron microscopy show that the micro-cracks initiate preferentially at the interface between long-period stacking order structures andα-Mg matrix and extend along the basal plane ofα-Mg.The massive long-period stacking order structures distributed at grain boundaries impede the transgranular propagation of cracks.

HIGH CYCLE FATIGUE PROPERTIES OF NICKEL-BASE ALLOY 718

The fatigue properties of nickel-base Alloy 718 with fine- and grain-coarse grains were investigated. In the fine-grain alloy, the fatigue strength normalized by the tensile strength was 0.51 at 107 cycles. In contrast, the fatigue strength of the coarse-grain alloy was 0.32 at the same cycles, although the fatigue strengths in the range from 103 to 105 cycles are the same for both alloys. The fracture appearances fatigued at around 106 cycles showed internal fractures originating from the flat facets of austenite grains for both alloys. The difference in fatigue strength at 107 cycles between the fine- and coarse-grain alloys could be explained in terms of the sizes of the facets from which the fractures originated.

EFFECTS OF HIGH TEMPERATURE PRETREATMENTS ON HIGH TEMPERATURE FRACTURE BEHAVIOR OF SiC-C/SiC

The samples made from a SiC-C/SiC composite were pretreated in AT under creep, fatigue, creep and fatigue interaction, as well as in dry oxygen and wet oxygen under fatigue at 1300℃for 15 hours. The fracture behaviors of the pretreated samples were investigated at 1300℃. The loading-strain curves and the microstructures of the sample were compared with each other. The various of high temperature tensile behaviors was attributed to the different microstructures resulted from different high temperature pretreatments.

EFFECT OF MICROSTRUCTURE OF ALUMINIDE COATING ON HIGH-CYCLE FATIGUE PERFORMANCE OF COATING-SUPERALLOY SYSTEM

The high-cycle fatigue performance of different microstructures of aluminide coating- superalloy system has been studied at 900℃.The single phase coating of coarse equiaxial grain NiAI(β)has unfavorable eJfect onJatigue life of the coating-superalloy.The fatigue life may shorten if the coating of NiAl(β)was an enrichment of coarse refractory metal grains. While an improvement can be made by dispersing numerous secondary phase particles such as extreme.fine γ′,quasi-σ-phase and others.

Intermediate Temperature Fatigue Induced Precipitation and Associated Corrosion in CrMnFeCoNi High Entropy Alloy

Understanding the corrosion behavior of high entropy alloys(HEAs)after intermediate temperature fatigue is critical to prevent their catastrophic failures from the reduction of corrosion resistance.Here,we investigated the corrosion behavior of CrMnFeCoNi HEA after 500℃ fatigue test with strain amplitudes of 0.2%and 0.5%.The intermediate temperature fatigue induced two types of precipitates,which were determined as Cr-richσphase and NiMn-rich L10 phase.Higher strain amplitude not only promoted precipitates generations but also spread the nucleation sites from intergranular to both intergranular and intragranular.Furthermore,we found that the deterioration in corrosion resistance of the alloy was derived from the increase of precipitates,which destroyed the stability of the passive film.The above results revealed that intermediate temperature fatigue impaired the stabilization of the solid solution state and subsequent corrosion resistance of CrMnFeCoNi HEA,where the higher strain amplitude led to more precipitates and more severe corrosion.

COMPARISON OF FATIGUE AND CREEP BEHAVIOR BETWEEN 2D AND 3D-C/SiC COMPOSITES

The differences of tension-tension fatigue and tensile creep characters of 2D-C/SiC and 3D-C/SiC composites have been scrutinized to meet the engineering needs. Experiments of tension-tension fatigue and tensile creep are carried out under vacuum high temperature condition. All of the high temperature fatigue curves are flat; the fatigue curves of the 2D-C/SiC are flatter and even parallel to the horizontal axis. While the tension-tension fatigue limit of the 3D-C/SiC is higher than that of the 2D-C/SiC, the fiber pullout length of the fatigue fracture surface of the 3D-C/SiC is longer than that of the 2D-C/SiC, and fracture morphology of the 3D-C/SiC is rougher, and pullout length of the fiber tows is longer. At the same time the 3D-C/SiC has higher tensile creep resistance. The tensile curve and the tensile creep curve of both materials consist of a series of flat step. These phenomena can be explained by the non-continuity of the damage.

MAGNETIC PROPERTY CHANGE IN AN AUSTENITIC STAINLESS STEEL SUBJECTED TO DAMAGE AT ELEVATED TEMPERATURE-MICROSTRUCTURE RESPONSIBLE FOR MAGNETIC PROPERTY

It has been found that magnetic property changes in austenitic stainless steel subjected to creep at high temperature. The change of magnetic property is mainly due to decrease the chromium concentration in the vicinity of grain boundary and transform into martensite in the area. However this result is for short-term creep. It is necessary to evaluate the long-term creep in order to develop non-destructive technique for plants. Moreover it is important to evaluate the fatigue. The change of magnetic property for those damages at high temperatures is investigated. The transformation into martensite is observed for both the long-term creep and fatigue. The magnetic regions are observed in sever deformed area and near crack. Then the formation of magnetic phases is related to the damage. The damage at high temperature can be universally evaluated.

孔挤压强化对GH4169孔结构高温疲劳裂纹扩展行为影响研究

为探究孔挤压强化对镍基高温合金GH4169孔结构疲劳裂纹扩展行为的影响规律,针对孔挤压强化后螺栓孔特征模拟件开展残余应力松弛及疲劳裂纹扩展试验。结果表明,经热松弛稳定后,孔边依旧可以保持至少400 MPa的残余压应力水平,且孔壁表面残余应力松弛幅度高于内部;孔挤压强化后疲劳裂纹扩展速率降低一个数量级,随着相对挤压量增加,疲劳裂纹扩展速率降低幅度增大,疲劳裂纹扩展寿命增幅为4~22倍。结合有限元仿真及断口分析,孔挤压强化对疲劳裂纹扩展的影响机制可归结为松弛至稳定后的残余压应力抵消部分外载应力,导致有效应力强度因子范围减小,使得疲劳裂纹扩展速率降低。基于该机制,利用残余应力修正的Walker模型对疲劳裂纹扩展寿命进行预测,误差在1.8倍分散带以内。

Testing and evaluation for fatigue crack propagation of Ti-6Al-4V/ELI and 7050-T7452 alloys at high temperatures

This paper seeks to evaluate crack propagation properties and residual lives of metallic alloys subjected to fatigue loading at room and high temperatures. Fatigue crack growth tests were performed on Ti-6Al-4V/ELI and 7050-T7452 subjected to constant-amplitude and actual randomspectra loading at room temperature of about 25 ℃ and at high temperatures of 250 ℃ and 150 ℃ to determine their crack growth properties and residual lives. The damage mode and mechanisms at high temperature were compared with those at room temperature on the basis of the results of fractographic analysis. Temperature-dependent residual lives under actual random-spectra load history were evaluated based on a modified accumulation damage rule accounting for the load interaction.Good correlation was achieved between the predictions and actual experiments, demonstrating the practical and effective use of the proposed method.

High-temperature failure mechanism and defect sensitivity of TC17 titanium alloy in high cycle fatigue

Crack initiation is an essential stage of fatigue process due to its direct effect on fatigue failure.However,for titanium alloys in high-temperature high cycle fatigue(HCF),the crack initiation mechanisms remain unclear and the understanding for the defect sensitivity is also lacking.In this study,a series of fatigue tests and multi-scale microstructure characterizations were conducted to explore the high-temperature failure mechanism,and the coupled effect of temperature and defect on TC17 titanium alloy in HCF.It was found that an oxygen-rich layer(ORL)was produced at specimen surface at elevated temperatures,and brittle fracture of ORL at surface played a critical role for surface crack initiation in HCF.Besides,internal crack initiation with nanograins at high temperatures was a novel finding for the titanium alloy.Based on energy dispersive spectroscopy,electron backscatter diffraction and transmission electron microscope characterizations,the competition between surface and internal crack initiations at high temperatures was related to ORL at surface and dislocation resistance in inner microstructure.The fatigue strengths of smooth specimens decreased at elevated temperatures due to the lower dislocation resistance.While the fatigue strengths of the specimens with defect were not very sensitive to the temperatures.Finally,a fatigue strength model considering the coupled effect of temperature and defect was proposed for TC17titanium alloy.